Electrical and Magnetic Properties Modification in Heavy Ion Irradiated Nanograin NixCo(3-x)O4 Films

Reactively sputtered NixCo(3-x)O4 films (x = 1.5, 1.0, and 0.75) were grown and subsequently irradiated with 5.5 MeV Si+ ions to investigate effects of lattice-site and charge state distribution. Films were characterized before and after irradiation by X-ray diffraction, X-ray photoemission spectroscopy, Rutherford backscattering spectroscopy, electric resistivity measurements, and temperature-dependent AC and DC magnetometry. Results indicate that ion irradiation induces oxygen loss, partial reduction of nickel, and an increase in both low-temperature ferrimagnetism and room-temperature conductivity. Frequency-dependent AC magnetic susceptibility measurements indicate a spin-glass-like transition at low temperature which moves to higher temperature after irradiation. Significance of the charge transfer for magnetism and conduction in a mixed spinel with Co2+, Co3+, Ni2+, and Ni3+ in tetrahedral and octahedral sites is discussed.